Facilitating the dry reforming of methane with interfacial synergistic catalysis in an Ir@CeO2−x catalyst

The dry reforming of methane provides an attractive route to convert greenhouse gases (CH4 and CO2) into valuable syngas, so as to resolve the carbon cycle and environmental issues. However, the development of high-performance catalysts remains a huge challenge. Herein, we report a 0.6% Ir/CeO2−x catalyst with a metal-support interface structure which exhibits high CH4 (~72%) and CO2 (~82%) conversion and a CH4 reaction rate of ~973 μmolCH4 gcat−1 s−1 which is stable over 100 h at 700 °C. The performance of the catalyst is close to the state-of-the-art in this area of research. A combination of in situ spectroscopic characterization and theoretical calculations highlight the importance of the interfacial structure as an intrinsic active center to facilitate the CH4 dissociation (the rate-determining step) and the CH2* oxidation to CH2O* without coke formation, which accounts for the long-term stability. The catalyst in this work has a potential application prospect in the field of high-value utilization of carbon resources.

The dry reforming of methane provides an attractive route to convert greenhouse gases (CH 4 and CO 2 ) into valuable syngas, so as to resolve the carbon cycle and environmental issues.However, the development of highperformance catalysts remains a huge challenge.Herein, we report a 0.6% Ir/CeO 2−x catalyst with a metal-support interface structure which exhibits high CH 4 (~72%) and CO 2 (~82%) conversion and a CH 4 reaction rate of ~973 μmol CH4 g cat −1 s −1 which is stable over 100 h at 700 °C.The performance of the catalyst is close to the state-of-the-art in this area of research.A combination of in situ spectroscopic characterization and theoretical calculations highlight the importance of the interfacial structure as an intrinsic active center to facilitate the CH 4 dissociation (the rate-determining step) and the CH 2 * oxidation to CH 2 O* without coke formation, which accounts for the long-term stability.The catalyst in this work has a potential application prospect in the field of highvalue utilization of carbon resources.
Owing to the increasing global warming and climate change issues, strategies for greenhouse gas reduction have drawn extensive interest from both fundamental research and industrial applications [1][2][3] .CO 2 and CH 4 are regarded as two predominant contributors to the greenhouse effect; therefore, their utilization and conversion to high-valueadded chemicals and fuels meet the demands for achieving large-scale carbon fixation, carbon emission reduction and carbon cycle [4][5][6][7] .One promising approach is to convert both CO 2 and CH 4 simultaneously through thermo-catalytic dry reforming of methane (DRM) reaction, which produces the syngas (H 2 and CO) as an important platform for alternatives of petroleum-derived fuels and valuable chemicals [8][9][10][11] .
Thermodynamically, the DRM reaction involves both C-H bond dissociation (439 kJ mol −1 ) and C=O bond hydrogenation (750 kJ mol −1 ) followed by subsequent formation of CO and H 2 , resulting in a highly endothermic process (ΔH 298K = 247 kJ mol −1 ) [12][13][14][15] .This normally requires a high energy consumption and rigorous reaction temperature (>800 °C) to maintain favorable catalytic activity, but suffers from serious catalyst deactivation due to nanoparticle agglomeration and carbon deposition [16][17][18] .In this case, great efforts have been focused on the exploration of catalysts towards DRM reaction, such as supported noble metals (e.g., Pt 19 , Ru 9,20 , and Pd 21,22 ) and non-noble metals (e.g., Ni [23][24][25] and Co 26 ) catalysts.Although considerable advances have been made, rational design and preparation of highly efficient catalysts to acquire high activity and stability simultaneously, still remain a big challenge.
In general, pure metal surfaces exhibit low reactivity towards methane dissociation and are prone to deactivation resulting from carbon deposition; whilst both experimental and theoretical studies have shown that C-H bond activation is more sensitive to coordinatively unsaturated metallic sites 12,27 .In this respect, the emerging strong metal-support interaction (SMSI) has demonstrated many appealing advantages, such as the interfacial structure and synergistic catalysis, which have attached widespread research interest in various heterogeneous reactions (e.g., CO 2 methanation and water gas shift reaction) [28][29][30][31] .The fine-tuning for SMSI has been successfully employed to optimize geometric/electronic structure of metal species at the interface [32][33][34] , which provides great opportunities to promote catalytic performance towards DRM reaction.On the one hand, the oxidic M δ+ metal species formed at the interfacial sites as an electron-acceptor, would reduce the T d symmetry structure of methane molecule and thus facilitate its activation dehydrogenation to CH x 6,12,20 .For instance, Pirovano et al. reported that Ni 2+ species promotes C−H bond dissociation at a lower temperature relative to metal Ni based on experiments and DFT calculations 35 .On the other hand, reducible supports (e.g., CeO 2 , ZrO 2 , and TiO 2 ), which renders a facile conversion between two oxidation states (e.g., Ce 4+ and Ce 3+ ), would stabilize oxidic M δ+ species via accommodating metal-to-support electron transfer 32,[36][37][38] .Meanwhile, the concomitant oxygen vacancies make a great contribution to elevate the activation adsorption of C=O group and facilitate the transformation of intermediates 34,39,40 .For example, Liu et al. reported the oxygen vacancies on CeO 2 surface serve as active center towards CO 2 hydrogenation to methanol, where the catalytic activity is highly correlated with the oxygen vacancies concentration 41 .This evokes us to design a suitable metal-support interface structure with synergistic catalysis effect, so as to simultaneously promote catalytic activity and stability for DRM reaction and further reveal the structure-property correlation at molecular/atomic scale.
Herein, we report an Ir nanoclusters supported on CeO 2 catalyst prepared through a facile impregnation-reduction method.HAADF-STEM, quasi in situ XPS and in situ XAFS confirm the formation of interface structure (Ir δ+ −O v −Ce 3+ ), whose concentration can be modulated via adjusting the Ir loading.The optimal catalyst 0.6% Ir/CeO 2−x (Fig. 1a) exhibits high conversions of CH 4 (~72%) and CO 2 (~82%) at 700 °C, with a CH 4 reaction rate of ~973 μmol CH4 g cat −1 s −1 ; and a 100 h stream-on-line test demonstrates a satisfactory stability without obvious deactivation.This is, to the best of our knowledge, preponderant to the state-of-the-art catalysts under similar reaction conditions.Kinetics studies verify that the dissociation of CH 4 is the rate-determining step in DRM reaction, whose activation energy decreases significantly by ~50 kJ mol   .In addition, the aberration-correction highangle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) was conducted to explore detailed structure of Ir/ CeO 2−x .As shown in Fig. 1e and f, a clear lattice fringe (~0.211 nm) indexed to Ir(111) plane is observed on the surface of CeO 2 for both the 0.2% and 0.6% Ir/CeO 2−x samples.Moreover, the energy dispersive spectroscopy (EDS) elemental mapping and elemental line scanning of 0.6% Ir/CeO 2−x sample (Fig. 1g−i) show a partial coating of CeO 2 on the surface of Ir cluster, indicating the formation of interfacial structure between Ir and CeO 2 .
Quasi in situ XPS spectra were performed to investigate the electronic structure of surface Ir species.As shown in Fig. 2a, the Ir/ Al 2 O 3 sample displays two peaks at 60.6 eV (Ir 4f 7/2 ) and 63.6 eV (Ir 4f 5/ 2 ) corresponding to the Ir 0 species.In contrast, for the four Ir/CeO 2−x samples, besides the same Ir 0 peaks, two additional strong peaks at 61.6 eV (Ir 4f 7/2 ) and 64.6 eV (Ir 4f 5/2 ) are found, which are attributed to the Ir δ+ species [44][45][46] .This indicates the electron transfer from Ir species to CeO 2 support at the interface via the electronic metal-support interaction (EMSI), which is absent in the Ir/Al 2 O 3 sample.With the increase of Ir content, the ratio of Ir δ+ /(Ir δ+ +Ir 0 ) declines gradually from 57% (0.2% Ir/CeO 2−x ) to 30% (3.0%Ir/CeO 2−x ) (Supplementary Table 2), as a result of the decreased Ir dispersion degree.Furthermore, in situ CO-DRIFTS is implemented to investigate the configuration of Ir species (Fig. 2d), from which a broad band centered at ~2020 cm −1 due to the linear CO at Ir 0 site is found for the Ir/Al 2 O 3 sample.Notably, in the case of Ir/CeO 2−x samples, both the linear adsorption of CO at Ir 0 (~2020 cm −1 ) and gem-dicarbonyl species adsorption at Ir δ+ (~2060 cm −1 ) are observed 44,45,47 .With the increment of Ir loading, according to the Gaussian peak fitting results, the relative peak intensity of Ir δ+ /(Ir δ+ +Ir 0 ) displays an obvious decrease from 0.2% Ir/ CeO 2−x (56%) to 3.0% Ir/CeO 2−x (23%) (Supplementary Table 3), in accordance with the variation tendency of Ir δ+ /(Ir δ+ +Ir 0 ) in the XPS results.X-ray absorption near-edge structure (XANES) measurements at normalized Ir L 3 -edge were implemented to analyze the electronic state and coordination fine structure.As shown in Fig. 2e, the white line peaks of Ir/CeO 2−x and Ir/Al 2 O 3 samples are located between Ir foil and IrO 2 reference, suggesting the existence of positively charged Ir species.Moreover, the intensity of white line declines gradually from 0.2% Ir/CeO 2−x to 3.0% Ir/CeO 2−x and then to Ir/Al 2 O 3 , indicating the decrease in the oxidation state of Ir δ+ species (reduced interfacial electron transfer) along with the increase in metallic Ir 0 .The Fourier transforms of the extended X-ray absorption fine spectra (EXAFS) in the R space (Supplementary Fig. 10) show that all these samples exhibit coexistent of Ir−O scattering (~1.5 Å) and Ir−Ir scattering (~2.5 Å).Accordingly, we conducted the linear combination fitting (LCF) of XANES (Fig. 2f) to determine the Ir species composition in these samples.The control sample Ir/Al 2 O 3 displays a low Ir 4+ atomic ratio of 27%.In contrast, the Ir 4+ is predominant for the Ir/CeO 2−x samples, in which the Ir 4+ atomic ratio of 0.2% and 0.6% Ir/CeO 2−x samples are 63% and 59% (Supplementary Table 4), respectively; whilst the Ir 0 plays a leading role for the 2.0% and 3.0% Ir/CeO 2−x samples, as a result of the increased particle size of Ir.The average oxidation state of iridium species is calculated based on the results from LCF analysis, which gives the following sequence: 0.2% Ir/CeO 2−x (+2.5) > 0.6% Ir/CeO 2−x (+2.4) > 2.0% Ir/CeO 2−x (+1.8) > 3.0% Ir/CeO 2−x (+1.7) > 0.6% Ir/ Al 2 O 3 (+1.1).

Catalytic evaluations
The preceding samples were evaluated for DRM under a gas hourly space velocity as high as 240000 mL g −1 h −1 at atmospheric pressure.As shown in Fig. 3a and b, both the CH 4 and CO 2 conversions as a function of reaction temperature show a positive correlation for these samples, duo to the strong endothermic characteristic.The control sample 0.6% Ir/Al 2 O 3 gives a normal catalytic performance towards DRM reaction; whilst the catalytic performance of Ir/CeO 2−x samples improve significantly.Notably, the catalytic activity exhibits a volcanic curve at each reaction temperature along with the increase of Ir loading: an increase from 0.2% Ir/CeO 2−x to 0.6% Ir/CeO 2−x (the maximum value) is present, followed by a slight descend to 2.0% Ir/CeO 2−x and 3.0% Ir/CeO 2−x .As for the optimal 0.6% Ir/CeO 2−x sample, both the CH 4 and CO 2 conversions reach up to the thermodynamic equilibrium, and the reaction rate is 3−20 times higher than previously reported studies under similar reaction conditions within 650−750 °C (Fig. 3c and Supplementary Table 5) 19,50 .Specifically, the 0.6% Ir/CeO 2−x catalyst exhibits high conversions of CH 4 (72%) and CO 2 (82%) with a CH 4 reaction rate of ~973 μmol CH4 g cat −1 s −1 at a relatively moderate temperature (700 °C), which are precedent to the state-of-the-art catalysts 6,7,9,16,20,24,25,[49][50][51] .In addition, the long-term stability test displays a rapid deactivation for the Ir/Al 2 O 3 sample within 15 h due to Ir agglomeration and carbon deposition at 700 °C (Supplementary Fig. 16 and 17).In contrast, both the CH 4 and CO 2 conversions of 0.6% Ir/CeO 2−x catalyst remain almost unchanged within 100 h on stream (Fig. 3d).Moreover, the used 0.6% Ir/CeO 2−x catalyst does not show obvious structural change compared with the fresh sample, verified by TEM, XPS and in situ CO-DRIFTS (Supplementary Fig. 18−20), indicating a satisfactory stability in DRM reaction.The results above demonstrate excellent performance of 0.6% Ir/CeO 2−x catalyst, which shows potential application in industrial applications.Furthermore, we performed kinetic studies on CH 4 and CO 2 activation as well as the rate-determining step in DRM system.Firstly, the effects of external and internal diffusion limitation have been eliminated under the aforementioned reaction conditions 34,50,52 .On this basis, the kinetic experimental data were studied via setting a stationary partial pressure of one reactant whilst changing the other partial pressure (Fig. 3e, f), and the obtained results were calculated for kinetic parameters and were shown in Supplementary Table 6.The reaction rate over 0.6% Ir/CeO 2−x catalyst displays a linear positive correlation with the partial pressure of CH 4 and CO 2 .Nevertheless, the calculated reaction order with respect to CH 4 (~0.67 and ~0.53) is significantly higher than that of CO 2 (~0.09 and ~0.07), indicating that the CH 4 activation is critical to the reaction kinetics, consistent with previous studies 50,52,53 .Moreover, the apparent activation energy (E a ) of CH 4 over 0.6% Ir/CeO 2−x is 91 kJ mol −1 , much larger than that of CO 2 (70 kJ mol −1 ) (Fig. 3g and Supplementary Fig. 22 and 23).The results verify that the CH 4 dissociation entails a higher energy barrier and serves as the rate-determining step in this catalytic system.Notably, the E a value on 0.6% Ir/CeO 2−x catalyst shows a marked decline by 35% relative to the 0.6% Ir/Al 2 O 3 sample, which indicates the interfacial sites play a critical role in activating reactant molecule.In addition, the intrinsic TOF of CH 4 is evaluated at a low conversion (below 15%, Fig. 3h), which gives a decrease order as follows: 0.2% Ir/CeO 2−x (168 mol CH4 mol Ir −1 s −1 ) > 0.6% Ir/CeO 2−x (163 mol CH4 mol Ir −1 s −1 ) > 2.0% Ir/CeO 2−x (122 mol CH4 mol Ir −1 s −1 ) > 3.0% Ir/ CeO 2−x (110 mol CH4 mol Ir −1 s −1 ).To further the reveal correlation of intrinsic active site and structure-property, the intrinsic TOF of CH 4 is plotted as a function of interfacial Ir δ+ concentration (based on the results of in situ CO-DRIFTS), from which an approximative linear relationship is present (Fig. 3i).Furthermore, a positive correlation between intrinsic TOF and surface Ce 3+ ratio (Supplementary Fig. 24) or surface oxygen vacancy ratio (Supplementary Fig. 25) is also demonstrated.The results above elucidate that the Ir δ+ −O v −Ce 3+ interfacial sites serve as the intrinsic active center towards DRM reaction, accounting for the prominent catalytic performance.
Catalytic mechanism.In situ/operando XANES of Ir L 3 -edge and Ce L 3edge combined with quasi in situ XPS were applied to reveal the dynamic variation of fine structure and electronic interaction at interfacial sites under the catalytic reaction.During the measurement, CH 4 and CO 2 was introduced into the reaction cell in turn, and the catalytic reaction was triggered at 700 °C via injecting the second reactant gas, so as to observe the formation and variation of interface structure (Ir δ+ −O v −Ce 3+ ).When CH 4 is introduced alone, the white line peaks of Ir and Ce shift close to the reference Ir foil and CeF 3 (Fig. 4a, d), respectively, indicating a decline in valence states of Ir and Ce (Fig. 4b, e).The corresponding variations in XPS spectra of Ir 4f and Ce 3d are also observed (Fig. 4c, f): the Ir δ+ /(Ir δ+ +Ir 0 ) ratio decreases whilst the Ce 3+ /(Ce 3+ + Ce 4+ ) ratio and O v increase.This implies the occurrence of CH 4 dissociation to CH x * species, which then combines with surface reactive O to generate more Ir δ+ −O v −Ce 3+ interface sites.After the injection of CO 2 , the white line peaks of Ir and Ce shift back to their original position, indicating the replenishment of O v by CO 2 .In Fig. 4a, d, as CO 2 is introduced alone, the white line peaks of Ir and Ce move close to the reference IrO 2 and CeO 2 , respectively (elimination of primary O v ); and XPS results show the increased valence states of Ir and Ce accompanied with reduced Ce 3+ /(Ce 3+ + Ce 4+ ) ratio and O v (Fig. 4b, e).Afterwards, the subsequent CH 4 flowing induces the recovery of Ir and Ce white line peaks to their original position, corresponding to the CH 4 dissociation assisted with surface reactive oxygen species.
In situ/operando DRIFTS experiments of reactants were carried out to further identify the intermediate species and monitor the evolution of dynamic reaction process at the interface structure Ir δ+ −O v −Ce 3+ (Fig. 5a−h).When CH 4 is introduced individually into the reactor at 700 °C, in addition to the gas phase CH 4 at ~3016 and ~1304 cm −1 , another two bands at ~1330 and ~1350 cm −1 corresponding to the deformation vibration of CH x * and CH 3 * are observed 4,20,51 , respectively, due to the activation adsorption and dissociation of CH 4 at interface Ir δ+ sites.Subsequently, with the injection of CO 2 , two strong peaks located at ~2360 and ~1550 cm −1 , as well as another broad one at ~3750−3550 cm −1 appear, which are attributed to the gas phase CO 2 , the monodentate carbonate species (HCOO*) and surface hydroxyl group (OH*), respectively (Fig. 5b−d and Supplementary Figs. 28, 29) 16,20,54,55 .Notably, another IR band assigned to the CH x O* species is found at ~1390 cm −1 , accompanied with the weakened bands of CH 4 and CH x * species 56,57 .This is probably due to the oxidation of CH x * by reactive oxygen species originating from CO 2 dissociation.In addition, three bands between ~2200 and ~2000 cm −1 are detected, which are ascribed to gaseous CO and adsorbed CO* at e, f Correlation of CH 4 or CO 2 partial pressure on the reaction rates of CH 4 and CO 2 .
g Kinetic studies and calculated activation energy (E a ) of CH 4 over various catalysts.h Intrinsic TOF over various catalysts within the catalytic dynamic range.i TOF as a function of interfacial Ir concentration calculated by CO-DRIFTS results.
Ir δ+ , respectively (Fig. 5c) [44][45][46] .Once the atmosphere is switched from the mixture gas (CO 2 and CH 4 ) to individual CH 4 , the bands of gas phase CO 2 weaken firstly, and then the bands assigned to CH x O*, HCOO*, OH* and CO species disappear gradually accompanied with the enhancement of CH 4 and CH x * peaks.This demonstrates the oxygen-containing species (CH x O*, HCOO*, OH*) serves as important intermediate, whose consumption can be reproduced by CO 2 at interface O v .
Next, we changed the study paradigm, in which CO 2 is injected into the reactor firstly under the same conditions.Accordingly, the bands assigned to CO 2 is observed (Supplementary Fig. 30 and Fig. 5e).With the subsequent flowing of CH 4 , the bands of CH 4 , CH x * and CH 3 * species are found (Fig. 5g, h), followed by the emergence of CH x O* and CO peaks as well as the weakened OH* band.This verifies the significance of CH x O* species originating from the reaction between CH x * and surface oxygen species, in accordance with the results of Fig. 5a−d.Operando investigations above (XAFS and DRIFTS) substantiate that the interface structure (Ir δ+ −O v −Ce 3+ ) serves as the intrinsic active center with a crucial synergistic effect: Ir δ+ promotes the activation adsorption of CH 4 molecule whilst CO 2 dissociation occurs at the Ce 3+ −O v site, followed by the formation of the key intermediate (CH x O* species).
To in-depth explore the decisive role of Ir δ+ −O v −Ce 3+ interfacial sites in the reaction process, DFT calculations were investigated on Ir 7 / CeO 2−x model based on the experimental results (Supplementary Fig. 31).As shown in Fig. 5i and Supplementary Fig. 32, firstly, CH 4 molecule undergoes adsorption at the interfacial Ir δ+ of Ir 7 /CeO 2−x (110) with a small adsorption energy (−0.03 eV); then, the C-H bond cleavage of CH 4 occurs to generate CH 3 * (TS1: 1.12 eV).Afterwards, the CH 3 * species experiences dehydrogenation process which shows an energy barrier of 1.43 eV, excluding the oxidation of CH 3 * to CH 3 O* with a large steric hindrance.Subsequently, two possible steps are involved: (1) CH 2 * oxidation to CH 2 O* and (2) CH 2 * dehydrogenation to CH*.However, the former displays a much lower energy barrier (TS3: 1.03 eV) than the latter (TS4: 1.56 eV), in agreement with the formation of CH x O* intermediate verified by the operando DRIFTS results.This step is crucial, which inhibits excessive decomposition of CH 2 * species to carbon deposition.The next dehydrogenation of CH 2 O* to CHO* (TS5) and CO (TS6) shows normal activation barriers of 0.63 and 0.73 eV, respectively.Finally, the produced CO undergoes desorption from the O v and the remaining four active hydrogen form into two H 2 molecules (Supplementary Fig. 32).Meanwhile, CO 2 molecule experiences dissociation adsorption at the O v on the surface with an adsorption energy of −1.85 eV and an energy barrier of 0.7 eV (Supplementary Fig. 33 and 34), with the formation of active oxygen species that participates in the CH 2 * oxidation to CH 2 O*.According to the calculation results, the dehydrogenation of CH 3 * species to CH 2 * gives the highest energy barrier (1.43 eV), which is determined as the ratedetermining step of DRM reaction, in accordance with the experimental results (Fig. 3e−f).In addition, a comparative study between Ir 7 / CeO 2−x and Ir 7 /Al 2 O 3 shows that the reaction energy barrier of ratedeterminingstep in the former case (1.43 eV) is significantly lower than the latter one (Supplementary Figs.35, 36: 2.88 eV), demonstrating the essential contributions of interface sites (Ir δ+ −O v −Ce 3+ ), in well agreement with the catalytic performance in Fig. 3a−h.
In summary, we report an Ir/CeO 2−x catalytic system with metal-support interface structure towards DRM reaction.The

Chemicals and materials
Analytical grade chemical reagents were purchased in Aladdin company and used directly without further purification, including: Ce(NO 3 ) 3

Preparation of catalysts
CeO 2 nanorods were prepared via a hydrothermal method reported by our group 42 .Typically, Ce(NO 3 ) 3 solution (0.4 M, 20 mL) and NaOH solution (6.8 M, 140 mL) were fully mixed with vigorous stirring for 30 min at room temperature.The obtained milky slurry was placed into a 200 mL sealed Teflon autoclave for 24 h at 100 °C.After filtering, washing thoroughly, and drying at 65 °C for 18 h, the sample was calcined in air at 500 °C with a heating rate of 10 °C min −1 for 4 h to obtain the CeO 2 nanorods support.CeO 2 (0.5 g) was dispersed into deionized water (20 ml) and H 2 IrCl 6 •6H 2 O aqueous solution (0.022 g mL −1 ; 0.105, 0.315, 1.050, 1.575 mL, respectively) was slowly dripped into above solution with vigorous stirring for various Ir loading samples.After 8 h of reaction, the resulting precipitate was centrifuged, washed thoroughly with deionized water and ethanol, followed by drying at 60 °C for 12 h.Before the DRM reaction, the sample was pre-treated at 750 °C for 3 h in a gaseous mixture of CH 4 and CO 2 (1:1, v/v; total flow rate: 50 mL min −1 ).As a reference, the Ir/ Al 2 O 3 sample was prepared via the same method described above by using Al 2 O 3 as the support, in which the pre-treated steps are in accordance with those of Ir/CeO 2−x samples.Characterizations X-ray diffraction (XRD) experiments were carried out with Bruker D8 Advance diffractometer.The elemental content was determined by Shimadzu ICPS-7500 equipment.The morphology and structure of catalysts were studied on JEOL JEM-2010 high-resolution transmission electron microscope.AC-HAADF-STEM images and EDS mapping data were performed on JEOL JEM-ARM200F.The CO pulses chemisorption experiments were conducted on Micromeritics Autochem II 2920.Quasi in situ XPS measurements were recorded on Kratos Axis Ultra DLD Instrument.The pre-treated sample was placed in a glove box and transferred into a sample rod in a N 2 atmosphere.In situ/Operando XAFS at Ir L 3 -edge and Ce L 3 -edge measurements were recorded at the beamline BL11B of the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences (CAS).In situ/operando DRIFTS were studied on a Bruker TENSOR II infrared spectrometer with a MCT detector.The detailed experimental methods are present in the Supplementary Information.

DFT calculations
The density functional theory (DFT) calculations based on firstprinciple methodology were investigated using the Vienna ab initio simulation package (VASP 5.4.4) 58,59 .Generalized gradient approximation (GGA) of PBE functional was applied to describe the exchange and correlation energy.Grimme's DFT-D3 method and projector augmented wave (PAW) method were employed to illustrate the effect of van der Waals interaction and to depict the core electrons, respectively 60,61 .The climbing image nudged elastic band (CI-NEB) method was employed to determine reaction transition states 62,63 .

Fig. 1 |
Fig. 1 | Microstructure and morphology studies of Ir/CeO 2−x samples.a Schematic illustration of Ir/CeO 2−x samples.b, d TEM and HR-TEM images of 0.6% Ir/CeO 2−x .c Particle size of various Ir/CeO 2−x and Ir/Al 2 O 3 samples determined by TEM.e, f High-resolution AC-HAADF-STEM images of 0.2% and 0.6% Ir/CeO 2−x , respectively.g, h AC-HAADF-STEM image and corresponding EDS mapping of 0.6% Ir/CeO 2−x .i Corresponding elemental line scanning of 0.6% Ir/CeO 2−x .

Fig. 2 |
Fig. 2 | Fine-structure characterizations of Ir/Al 2 O 3 and Ir/CeO 2−x samples.a-c Quasi in situ XPS of Ir 4f, Ce 3d an O 1s for Ir/Al 2 O 3 and Ir/CeO 2−x samples with various Ir loading.d In situ CO-DRIFTS spectra on the surface over Ir/Al 2 O 3 and

FreshFig. 4 |
Fig. 4 | Local coordination structure and surface structure of 0.6% Ir/CeO 2 during DRM reaction.a, b In situ/operando normalized XANES at Ir L 3 -edge and diagram of the linear combination fitting (LCF) results of 0.6% Ir/CeO 2−x with CH 4 , CO 2 and CH 4 + CO 2 treatment, respectively.d, e Ce L 3 -edge and diagram of the linear combination fitting (LCF) results of 0.6% Ir/CeO 2−x with CH 4 , CO 2 and CH 4 + CO 2 treatment, respectively.c, f Quasi in situ XPS spectra of Ir 4f and Ce 3d for the fresh 0.6% Ir/CeO 2−x and the same catalyst after CH 4 or CO 2 treatment at 700 °C.

Fig. 5 |
Fig. 5 | In situ/operando DRIFTS spectra and DFT calculations of DRM reaction on 0.6% Ir/CeO 2−x .In situ/operando DRIFTS spectra over 0.6% Ir/CeO 2−x at 700 °C after in-situ pretreatment and He purging, followed by exposure to: (a−d) first CH 4 atmosphere, subsequent CH 4 + CO 2 and then CH 4 atmosphere for 30 min, respectively; (e−h) first CO 2 atmosphere, subsequent CO 2 + CH 4 and then CO 2 atmosphere for 30 min, respectively.i Schematic illustration for DRM reaction at the interface of Ir/CeO 2−x .Ir, green; Ce, yellow; C, gray; O, crimson; H, white.The inset shows potential energy profile of CH 4 decomposition by Ir/CeO 2−x (110).'TS' represents a transition state.The black and orange numbers represent the adsorption energies and energy barriers of the elementary steps, respectively.